This invention relates to electric machines and more particularly to a winding end cap assembly for an electric machine.
Reluctance electric machines, such as motors and generators, typically include a stator that is mounted inside a machine housing and a rotor that is supported for rotation relative to the stator. Reluctance electric machines produce torque as a result of the rotor tending to rotate to a position that minimizes the reluctance of the magnetic circuit (and maximizes the inductance of the stator windings). The reluctance of the rotor is minimized when a pair of diametrically-opposed rotor poles are aligned with a pair of energized and diametrically-opposed stator poles. In synchronous reluctance electric machines, the windings are energized at a controlled frequency. In switched reluctance electric machines, the angular position of the rotor is detected. A drive circuit energizes the stator windings as a function of the sensed rotor position.
There are two distinct approaches for detecting the angular position of the rotor. In a xe2x80x9csensedxe2x80x9d approach, an external physical sensor senses the angular position of the rotor. For example, a rotor position transducer (RPT) with a hall effect sensor or an optical sensor physically senses the angular position of the rotor. In a xe2x80x9csensorlessxe2x80x9d approach, electronics that are associated with the drive circuit derive the angular rotor position without an external physical sensor. For example in the sensorless approach, rotor position is derived by measuring the back electromotive force (EMF) in an unenergized winding, by introducing diagnostic pulses into the energized and/or the unenergized windings and sensing the resulting electrical response, or by sensing other electrical parameters and deriving rotor angular position.
Conventional switched reluctance electric machines generally include a stator with a solid stator core and/or a laminated stator with a plurality of circular stator plates that are punched from a magnetically conducting material and that are stacked together. The stator plates define salient stator poles that project radially inward and inter-polar slots that are located between the adjacent stator poles. Winding wire is wound around the stator poles. As can be appreciated, increasing the number of winding turns and the slot fill increases the torque density of the electric machine. The stator poles of switched reluctance electric machines typically have parallel sides that do not inherently hold the winding wire in position. Tangs on radially inner ends of the stator poles have been provided to help maintain the winding wire on the stator poles with some limited success.
There are several conventional methods for placing the winding wire on the stator of a switched reluctance electric machine. The winding wire can be initially wound and transferred onto the stator poles. Alternately, needle winding can be used to wind the wire around the stator poles. Both methods tend to leave excess winding wire or loops around axial ends of the stator poles. While winding a large number of turns around the stator poles is good for machine performance, it is difficult to hold the winding wire in place during wrapping and forming of the windings. In addition, the position of winding wire on the stator poles varies from one stator pole to the next and from one electric machine to the next. In other words, the individual winding turns are positioned differently and the cross sectional pattern of the stator pole windings are different. As a result, the inductance and resistance of the stator poles and of the electric machines often vary from one stator pole to the next even though the same number of winding turns are used. Axially inserted wedges or top sticks have also been used between the stator poles to help position the windings with moderate success.
As previously mentioned above, drive circuits of the switched reluctance electric machines need the angular position of the rotor as an input. There are many problems that are associated with switched reluctance electric machines that employ the sensed approach. In the sensed approach, the RPT detects the angular position of the rotor with respect to the stator. The RPT typically includes a sensor board with one or more sensors and a shutter that is coupled to and rotates with the shaft of the rotor. The shutter includes a plurality of shutter teeth that pass through optical sensors as the rotor rotates.
Because the angular rotor position is critical to proper operation of a switched reluctance electric machine, sophisticated alignment techniques are used to ensure that the sensor board of the RPT is properly positioned with respect to the housing and the stator. Misalignment of the sensor board is known to degrade the performance of the electric machine. Unfortunately, utilization of these complex alignment techniques increases the manufacturing costs for switched reluctance electric machines equipped with RPTs.
The RPTs also increase the overall size of the switched reluctance electric machine, which can adversely impact machine and product packaging requirements. The costs of the RPTs often place switched reluctance electric machines at a competitive disadvantage in applications that are suitable for open-loop induction electric machines that do not require RPTs.
Another drawback with RPTs involves field servicing of the switched reluctance electric machines. Specifically, wear elements, such as the bearings, located within the enclosed rotor housing may need to be repaired or replaced. To reach the wear elements, an end shield must be removed from the housing. Because alignment of the sensor board is critical, replacement of the end shield often requires the use of complex realignment techniques. When the alignment techniques are improperly performed by the service technician, the sensor board is misaligned and the motor""s performance is adversely impacted.
In an effort to eliminate the RPTs and to reduce manufacturing costs and misalignment problems, it would be desirable to employ the sensorless techniques for sensing rotor position. Switched reluctance electric machines that employ the sensorless approach also have several problems. The sensorless approach detects the magnitude of the back-electromotive force (EMF) of an unenergized winding of the stator in the switched reluctance electric machine or employs diagnostic pulses that are output to energized and/or unenergized windings. The windings are commutated when the sensed EMF magnitude reaches a predetermined level. If diagnostic pulses are used, the windings are commutated when the proper electrical response is sensed. Several patents disclosing sensorless techniques for sensing rotor position in switched reluctance electric machines include U.S. Pat. No. 5,929,590 to Tang and U.S. Pat. No. 5,877,568 to Maes, et al. which are hereby incorporated by reference.
Despite the apparent advantages that are associated with the elimination of RPTs, the sensorless approach has achieved limited success due to the variable electrical characteristics of the stator windings. For example, one source of the variable electrical characteristics is caused by inconsistent placement of the individual winding turns on the stator poles during assembly. When the positions of the individual winding turns relative to the stator pole vary from one stator pole to the next, the electrical characteristics of the stator poles will likewise vary. During use, the windings may also tend to creep or move if they are not held in place. The variable electrical characteristics of the stator windings make it difficult to consistently identify rotor angular position from the relatively low back-EMFs. Likewise the electrical response of the stator windings to the diagnostic pulses is also adversely and inconsistently impacted by variable electrical characteristics.
While the design of switched reluctance electric machines is relatively mature, there are several areas requiring improvement. Specifically, it is desirable to improve the consistency of the electrical characteristics of the stator poles of switched reluctance electric machines. It is desirable to improve the torque density of switched reluctance electric machines through increased slot fill. By increasing the torque density, the size of the switched reluctance electric machine can be reduced for a given torque output or the size can be maintained with an increase in torque output. Electrical machines achieving higher torque density will allow designers of products equipped with switched reluctance electrical machines greater flexibility in product design that may lead to increased sales through product differentiation and/or improved profit margins.
It is also desirable to eliminate the need for RPTs in switched reluctance electric machines. It is also desirable to assemble the windings of the stator of a switched reluctance electric machine in a highly uniform and repeatable manner to improve the performance of sensorless switched reluctance electric machines by reducing variations in the inductance and resistance of the stator.
A winding end cap assembly according to the invention for an electric machine with a stator and stator poles includes first and second end caps that are connected to opposite axial end surfaces of one of the stator poles. A first inner winding retainer section extends axially to connect an inner end of the first end cap to an inner end of the second end cap.
In other features of the invention, a second inner winding retainer section extends axially to connect the inner end of the first end cap to the inner end of the second end cap. The first and second end caps and the first and second inner winding retainer sections define an annular channel for receiving and retaining winding wire that is wound around the first and second end caps, the stator pole and the first and second inner winding retainer sections. The winding end cap assembly further includes first and second outer retainer sections that connect either hub sections or outer sections of said first and second end caps together.
In another aspect of the invention, a switched reluctance electric machine includes a stator. The stator includes a plurality of stator segments assemblies each with a stator segment core. An end cap assembly includes first and second end caps that are positioned adjacent to opposite axial end surfaces of the stator segment core. A first inner winding retainer section extends axially to connect an inner end of the first end cap to an inner end of the second end cap.
In still another aspect of the invention, a stator segment assembly for a stator of a switched reluctance electric machine includes a stator segment core. The stator segment core includes a radially outer rim section and a tooth section that extends radially inwardly from a center portion of the radially outer rim section. An end cap assembly defines a continuous annular channel. The end cap assembly includes first and second end caps that are positioned adjacent to opposite axial end surfaces of the stator segment core. First and second inner winding retainer sections extend axially to connect inner ends of the first and second end caps together. The first and second inner winding retainer sections engage inner ends of the tooth section.
Still other aspects, objects, features and advantages will be apparent from the specification, the drawings and the claims that follow.